09-12-2015, 05:06 PM
O.K. Mark, I just read your comments on voltage and series resistors - I'll probably print out the page here at the library and then stick it in my textbook for reference. I hope you understand I'm not being rude when I seem to delay responding directly to explanations because sometimes I have to do this later on. Let's say I get about 4 or 5 posts but only have an hour and bit, plus e-mails to sort through.... Anyway, thanks for your help and patience.
I had a pretty late night trying to advance further because now there is some similar information on testing resistors. This bit was tricky. The fundamental point that gave me pause for thought is that, back in the early Forties, the radio engineers weren't as spoiled as we are. Checking if a resistor was O.K. wasn't simply a case of sticking your DMM on, say, 200 K, and probing a resistor. Times were hard and testing equipment was costly so pocket Milliameters were vital.
Yes, as you said in your post, current limitations were an issue. The book states (from memory):
1000 R (up to 30 Milliamps on tester)
10000 R (up to 10 Milliamps on tester)
100,000 R (up to 5 Milliamps).
So, for the sheer hell of it, this is what I did in various imaginary scenarios:
With the aid of a calculator, I imagined I needed to test a 20,000 R resistor. The voltage according to the book was 100 volts. According to my calculation, the meter ought to deflect to 0.005 or 5 Milliamps because 100 divided by 20,000 is 0.005.
However, here is the punch line: Suppose the resistor is knackered? Suppose it's only able to offer a resistance of a mere 10,000 R? Or suppose the resistance is so low you could get 500 milliamps kicked out. Well, apparently the done thing was to start off with a much smaller voltage and work out the whole scenario through proportion. I won't risk giving any examples in case I make a typo or mistake and jar a few nerves but basically I plotted out a few expected current readings at, say, 30 volts. Then imagined the resistor was damaged and how that could be detected by proportion.
Of course, why bother? All I can say is maybe it gives a kind of perspective on how different things were back then. Not only did most people wear a hat on the street but a fair few seemingly relied upon a pocket milliameter. Even worse, no calculator so they probably did it all on paper!
Lastly, any mistakes or typos here, please all be aware it may happen and if you point it out, I'll go over it again!
Finally, how much is a pocket milliameter of the Forties era? As well as Avo 7's and 8's?
I had a pretty late night trying to advance further because now there is some similar information on testing resistors. This bit was tricky. The fundamental point that gave me pause for thought is that, back in the early Forties, the radio engineers weren't as spoiled as we are. Checking if a resistor was O.K. wasn't simply a case of sticking your DMM on, say, 200 K, and probing a resistor. Times were hard and testing equipment was costly so pocket Milliameters were vital.
Yes, as you said in your post, current limitations were an issue. The book states (from memory):
1000 R (up to 30 Milliamps on tester)
10000 R (up to 10 Milliamps on tester)
100,000 R (up to 5 Milliamps).
So, for the sheer hell of it, this is what I did in various imaginary scenarios:
With the aid of a calculator, I imagined I needed to test a 20,000 R resistor. The voltage according to the book was 100 volts. According to my calculation, the meter ought to deflect to 0.005 or 5 Milliamps because 100 divided by 20,000 is 0.005.
However, here is the punch line: Suppose the resistor is knackered? Suppose it's only able to offer a resistance of a mere 10,000 R? Or suppose the resistance is so low you could get 500 milliamps kicked out. Well, apparently the done thing was to start off with a much smaller voltage and work out the whole scenario through proportion. I won't risk giving any examples in case I make a typo or mistake and jar a few nerves but basically I plotted out a few expected current readings at, say, 30 volts. Then imagined the resistor was damaged and how that could be detected by proportion.
Of course, why bother? All I can say is maybe it gives a kind of perspective on how different things were back then. Not only did most people wear a hat on the street but a fair few seemingly relied upon a pocket milliameter. Even worse, no calculator so they probably did it all on paper!
Lastly, any mistakes or typos here, please all be aware it may happen and if you point it out, I'll go over it again!
Finally, how much is a pocket milliameter of the Forties era? As well as Avo 7's and 8's?
